Batteries and electrochemical capacitors (ECs) stand at opposite ends of the spectrum in terms of their power and energy densities. Batteries store energy through electrochemical reactions and can exhibit high energy densities (on the order of 20 to 150 Wh/kg), whereas ECs, which store charge in electrochemical double layers (EDLs), can only achieve values of 4 to 5 Wh/kg. However, because ion flow is faster than redox reactions, ECs can deliver much higher power densities. ECs are also generally maintenance free and display a longer shelf and cycle life, so they are often favored in many electronic applications.
An EC that combines the power performance of capacitors with the high energy density of batteries would represent a major advance in energy storage technology, but this requires an electrode with higher and more accessible surface area than that of conventional EC electrodes while maintaining high conductivity. Carbon-based materials are attractive in this regard because of their mechanical and electrical properties as well as exceptionally high surface area. Recently, the intrinsic capacitance of single layer graphene was reported to be ˜21 μF/cm2; this value now sets the upper limit for EDL capacitance for all carbon-based materials. Thus, ECs based on carbon-based materials could, in principle, achieve an EDL capacitance as high as ˜550 F/g if their entire surface area could be used.
Currently, carbon-based materials derived from graphite oxide (GO) can be manufactured on the ton scale at low cost, making them potentially cost effective materials for charge storage devices. Although these carbon-based materials have shown excellent power density and life-cycle stability, their specific capacitance (130 F/g in aqueous potassium hydroxide and 99 F/g in an organic electrolyte) still falls far below the theoretical value of ˜550 F/g calculated for a single layer of carbon. A variety of other carbon-based materials derived from GO have also been used, yet the values of specific capacitance, energy density, and power density have remained lower than expected. These effects are often attributed to the restacking of carbon sheets during processing as a result of the strong sheet-to-sheet van der Waals interactions. This reduction in the specific surface area of single layer carbon accounts for the overall low capacitance. In addition, these ECs exhibited relatively low charge/discharge rates, which precludes their use for high power applications. Recently, EC devices composed of curved graphene, activated graphene, and solvated graphene have demonstrated enhanced performance in terms of energy density. However, further improvements in energy density are needed that do not sacrifice high power density. In particular, the production of mechanically robust carbon-based electrodes with large thicknesses (˜10 μm or higher) and high surface-to-volume ratio in a binder free process would result in high power and high energy density ECs.
In the pursuit of producing high quality bulk carbon-based devices such as ECs and organic sensors, a variety of syntheses now incorporate graphite oxide (GO) as a precursor for the generation of large scale carbon-based materials. Inexpensive methods for producing large quantities of GO from the oxidation of graphitic powders are now available. In addition, the water dispersibility of GO combined with inexpensive production methods make GO an ideal starting material for producing carbon-based devices. In particular, GO has water dispersible properties. Unfortunately, the same oxygen species that give GO its water dispersible properties also create defects in its electronic structure, and as a result, GO is an electrically insulating material. Therefore, the development of device grade carbon-based films with superior electronic properties requires the removal of these oxygen species, re-establishment of a conjugated carbon network, as well as a method for controllably patterning carbon-based device features.
Methods for reducing graphite oxide have included chemical reduction via hydrazine, hydrazine derivatives, or other reducing agents, high temperature annealing under chemical reducing gases and/or inert atmospheres, solvothermal reduction, a combination of chemical and thermal reduction methods, flash reduction, and most recently, laser reduction of GO. Although several of these methods have demonstrated relatively high quality graphite oxide reduction, many have been limited by expensive equipment, high annealing temperatures and nitrogen impurities in the final product. As a result, of these difficulties, a combination of properties that includes high surface area and high electrical conductivity in an expanded interconnected carbon network has remained elusive. In addition, large scale film patterning via an all-encompassing step for both GO reduction and patterning has proven difficult and has typically been dependent on photo-masks to provide the most basic of patterns. Therefore, what is needed is an inexpensive process for making and patterning an interconnected corrugated carbon-based network (ICCN) having a high surface area with highly tunable electrical conductivity and electrochemical properties.